1. Field of the Invention
The present invention relates to Coriolis gyros. More particularly, this invention pertains to a method for determining the zero-point error of a Coriolis gyro.
2. Description of the Prior Art
Coriolis gyros (also known as “vibration gyros”) are increasingly employed for navigation. Such devices include a mass system that is caused to oscillate. Such oscillation is generally a superimposition of a large number of individual oscillations. The individual oscillations of the mass system are initially independent of one another and each may be regarded in the abstract as a “resonator”. At least two resonators are required for operation of a vibration gyro. A first resonator is artificially stimulated to oscillate, with such oscillations referred to below as a “stimulation oscillation”. A second resonator is stimulated to oscillate only when the vibration gyro is moved or rotated. That is, Coriolis forces occur which couple the first resonator to the second resonator, draw energy from the stimulation oscillation of the first resonator, and transfer the energy to the read oscillation of the second resonator. The oscillation of the second resonator is referred to below as the “read oscillation”. In order to determine movement (in particular rotation) of the Coriolis, the read oscillation is tapped off and a corresponding read signal (e.g. the tapped-off read oscillation signal) is analyzed to determine whether any changes occurred in the amplitude of the read oscillation that measures rotation of the Coriolis gyro. Coriolis gyros may be in the form of either an open loop or a closed loop system. In a closed loop system, the amplitude of the read oscillation is continuously reset to a fixed value (preferably zero) by control loops.
FIG. 2 is a schematic diagram of a closed loop Coriolis gyro 1. The gyro 1 has a mass system 2 that can be caused to oscillate and is referred to below as a resonator 2 (in contrast to the “abstract” resonators, mentioned above, which represent individual oscillations of the “real” resonator). As already mentioned, the resonator 2 may be regarded as a system composed of two “resonators” (a first resonator 3 and a second resonator 4). Each of the first and second resonators 3, 4 is coupled to a force transmitter (not shown) and to a tapping-off system (not shown). Noise produced by the force transmitter and the tapping-off system is indicated schematically by noise 1 (reference symbol 5) and noise 2 (reference symbol 6).
The Coriolis gyro 1 includes four control loops. A first control loop is employed for controlling the stimulation oscillation (i.e. the frequency of the first resonator 3) at a fixed frequency (resonant frequency). The first control loop has a first demodulator 7, a first low-pass filter 8, a frequency regulator 9, a VCO (voltage controlled oscillator) 10 and a first modulator 11. A second control loop controls the stimulation oscillation at a constant amplitude and includes a second demodulator 12, a second low-pass filter 13 and an amplitude regulator 14.
Third and fourth control loops are used for resetting forces that stimulate the read oscillation. The third control loop includes a third demodulator 15, a third low-pass filter 16, a quadrature regulator 17 and a second modulator 18. The fourth control loop comprises a fourth demodulator 19, a fourth low-pass filter 20, a rotation rate regulator 21 and a third modulator 22.
The first resonator 3 is stimulated at its resonant frequency ω1. The resultant stimulation oscillation is tapped off, demodulated in phase by means of the first demodulator 7, and a demodulated signal component passed to the first low-pass filter 8 that removes the sum frequencies. The tapped-off signal is referred to below as the tapped-off stimulation oscillation signal. An output from the first low-pass filter 8 is supplied to a frequency regulator 9 that controls the VCO 10 as a function of the applied signal so that the in-phase component essentially tends to zero. For this, the VCO 10 sends a signal to the first modulator 11, which controls a force transmitter so that a stimulation force is applied to the first resonator 3. When the in-phase component is zero, the first resonator 3 oscillates at its resonant frequency ω1. It should be mentioned that all of the modulators and demodulators are operated on the basis of resonant frequency ω1.
The tapped-off stimulation oscillation signal is also passed to the second control loop and demodulated by the second demodulator 12. The output of the second demodulator 12 is passed through the second low-pass filter 13, whose output signal is, in turn, applied to the amplitude regulator 14. The amplitude regulator 14 controls the first modulator 11 as a function of such signal and of a nominal amplitude transmitter 23 such that the first resonator 3 oscillates at a constant amplitude (i.e. the stimulation oscillation has constant amplitude).
As has already been mentioned, movement or rotation of the Coriolis gyro 1 results in Coriolis forces (indicated by the FC·cos(ω1·t) in the drawing) that couple the first resonator 3 to the second resonator 4, causing the second resonator 4 to oscillate. A resultant read oscillation at frequency ω2 is tapped off so that a corresponding tapped-off read oscillation signal (read signal) is supplied to both the third and fourth control loops. In the third control loop, this signal is demodulated by means of the third demodulator 15, the sum frequencies removed by the third low-pass filter 16, and the low-pass-filtered signal supplied to quadrature regulator 17 whose output is applied to the third modulator 22 so that corresponding quadrature components of the read oscillation are reset. Analogously, the tapped-off read oscillation signal is demodulated in the fourth control loop by means of a fourth demodulator 19. It then passes through a fourth low-pass filter 20 and the filtered signal is applied to a rotation rate regulator 21. The output of the rotation rate regulator 21 is proportional to the instantaneous rotation rate and is passed as the rotation rate measurement to a rotation rate output 24 and to the second modulator 18, which resets the corresponding rotation rate components of the read oscillation.
A Coriolis gyro 1 as described above can be operated in either a double-resonant form or in a form in which it is not double-resonant. When the Coriolis gyro 1 is operated in a double-resonant form, the frequency of ω2 of the read oscillation is approximately equal to the frequency ω1 of the stimulation oscillation. In contrast, when it is operated in a form in which it is not double-resonant, the frequency ω2 of the read oscillation differs from the frequency ω1 of the stimulation oscillation. In the case of double-resonance, the output signal from the fourth low-pass filter 20 contains information about the rotation rate, while, when it is not operated in double-resonant form, the output signal from the third low-pass filter 16 contains the rotation rate information. A doubling switch 25 which selectively connects the outputs of the third and fourth low-pass filters 16, 20 to the rotation rate regulator 21 and to the quadrature regulator 17 is provided for switching between the double-resonant and non-double resonant modes.
Due to inevitable manufacturing tolerances, it is not possible to avoid the force transmitter system that stimulates the first resonator (stimulation oscillation) while also slightly stimulating the second resonator (read oscillation). The tapped-off read oscillation signal thus includes a part due to Coriolis forces and a part (undesirably) due to manufacturing tolerances. The undesirable part results in the Coriolis gyro having a zero-point error whose magnitude is not possible to distinguish between the two parts when tapping off the tapped-off read oscillation signal.